The present invention relates to variable displacement compressors that are used in vehicle air conditioners and that change the inclination of a drive plate for controlling the displacement.
FIGS. 5 and 6 illustrate a part of such a compressor. This compressor includes housing 105, a drive shaft 102, a rotor 101 fixed to the shaft 102 and a swash plate 103, which functions as a drive plate. A shaft hole 103a is formed in the center of the swash plate 103. The drive shaft 102 extends through the hole 103a. A predetermined clearance exits between the shaft 102 and the hole 103a. The housing 105 has cylinder bores 105a. A piston 104 is housed in each cylinder bore 105a and is coupled to the periphery of the swash plate 103 by a pair of shoes 106.
A hinge mechanism 107 is located between the rotor 101 and the swash plate 103. The hinge mechanism 107 includes a pair of guide pins 108, which are formed on the swash plate 103, and a support arm 109, which is formed on the rotor 101. Each guide pin 108 has a ball 108a at its distal end. A pair of guide holes 109a are formed in the support arm 109. The axis of each guide hole 109a is inclined relative to a plane perpendicular to the axis L of the drive shaft 102. The ball 108a of each guide pin 108 is inserted in the corresponding guide hole 109a of the support arm 109. The inner wall 109b of each guide hole 109a functions as a guide surface that guides the movement of the associated ball 108a.
The rotor 101 and the hinge mechanism 107 cause the swash plate 103 to integrally rotate with the drive shaft 102. The rotation of the swash plate 103 is converted into linear reciprocation of the pistons 104 by the shoes 106. The pistons 104 compress refrigerant gas drawn into the cylinder bores 105a and discharge the compressed gas from the cylinder bores 105a. The swash plate 103 includes a top dead center point D1 that positions each piston 104 at the top dead center in the associated cylinder bore 105a. The swash plate 103 also includes a bottom dead center point D2 that positions each piston 104 at the bottom dead center in the associated cylinder bore 105a. As shown in FIGS. 5 and 6, a piston 104 that corresponds to the top dead center point D1 is located at the top dead center. When the swash plate 103 is rotated by 180 degrees from the states of FIGS. 5 and 6, the illustrated piston 104 will correspond to the bottom dead center point D2 and be located at the bottom dead center.
The hinge mechanism 107 guides the swash plate 103 to tilt between the maximum inclination shown in FIG. 5 and the minimum inclination shown in FIG. 6. Tilting of the swash plate 103 causes the guide balls 108a of the guide pins 108 to slide in the guide holes 109a. Also, the tilting causes the swash plate 103 to slide on the drive shaft 102. The clearance between the hole 103a of the swash plate 103 and the drive shaft 102 allows the swash plate 103 to smoothly move on the drive shaft 102. When the inclination of the swash plate 103 is changed, the bottom dead center of each piston 104 is changed while its top dead center remains unchanged. Accordingly, the stroke of the pistons 104 is changed. The changes in the piston stroke vary the displacement of the compressor.
When each piston 104 is located at the top dead center, the top clearance of each piston 104 (the distance between the end of the piston 104 and a valve plate, which is not illustrated in the drawings) is preferably as close to zero as possible and constant. Such a top clearance allows the compressor to constantly operate with a high compression efficiency at any given inclination angle of the swash plate 103. The hinge mechanism 107 is therefore designed to maintain a predetermined top dead center position for each piston 104 at any given inclination angle of the swash plate 103.
When at the maximum inclination position as shown in FIG. 5, the swash plate 103 maximizes the stroke of the pistons 104 thereby maximizing the compression ratio of refrigerant gas. At this time, the gas compressing process generates relatively large reactive forces. The reactive forces result in a relatively large force F. The force F acts on the inner wall 109b of the guide hole 109a through the pistons 104, the swash plate 103 and the balls 108a of the guide pins 108. The inner wall 109b is inclined relative to a plane perpendicular to the axis L of the drive shaft 102. Therefore, the force F is divided into a component F1 that is parallel to the inner wall 109b and a component F2 that is normal to the inner wall 109b.
The component F1 is directed away from the axis L of the drive shaft 102 and therefore moves the swash plate 103 upward as viewed in FIG. 5. In other words, the component F1 moves the top dead center point D1 of the swash plate 103 away from the axis L of the drive shaft 102. As a result, as shown in FIG. 5, a specific point (a point radially corresponding to the bottom dead center point D2) of the hole 103a is pressed against the drive shaft 102. In other words, the swash plate 103 is rotated integrally with the drive shaft 102 with the point corresponding to the point D2 pressed against the shaft 102. In this state, the swash plate 103 does not radially chatter relative to the drive shaft 102.
When at the minimum inclination position as shown in FIG. 6, the swash plate 103 minimizes the stroke of the pistons 104 thereby minimizing the compression ratio of refrigerant gas. This diminishes the force F generated by compression reaction forces acting on the pistons 104. The component F1 is decreased, accordingly. The force that moves the top dead center point D1 away from the axis L of the drive shaft 102 is thus weaker. When this force is smaller than the force of gravity acting on the swash plate 103, the swash plate 103 is moved by gravity.
For example, in FIG. 6, the swash plate 103 is moved in the direction of gravity (downward as viewed in FIG. 6). That is, a point on the inner wall of the hole 103a radially corresponding to the top dead center point D1 contacts the drive shaft 102. When the swash plate 103 is rotated by 180 degrees from the state of FIG. 6, a point on the inner wall of the hole 103a corresponding to the bottom dead center point D2 contacts the drive shaft 102. In other words, rotation of the swash plate 103 keeps changing the point on the inner wall of the hole 103a that contacts the drive shaft 102, thereby radially chattering the swash plate 103 with respect to the drive shaft 102. This results in noise and vibration.
When the swash plate 103 is at the maximum inclination position as shown in FIG. 5, a point on the inner wall of the hole 103a corresponding to the bottom dead center point D2 constantly contacts the drive shaft 102. On the other hand, when the swash plate 103 is at the minimum inclination position as shown in FIG. 6, a point on the inner wall of the hole 103a corresponding to the top dead center point D1 contacts the drive shaft 102. That is, the radial position of the swash plate 103 with respect to the axis L of the drive shaft 102 is different in the state of FIG. 5 from the state of FIG. 6. This causes the top dead center position of the piston 104 illustrated in FIG. 6 to be closer to the valve plate (not shown) than the top dead center position of the piston 104 illustrated in FIG. 5. In other words, the top dead center of FIG. 6 is moved toward the top of the cylinder bore 105a (to the right as viewed in the drawings) from the top dead center of FIG. 5. In other words, the top clearance of the piston 104 when at the top dead center is unstable. This causes unstable compression of refrigerant gas. Further, the pistons 104 must be prevented from contacting the valve plate. The top clearance of the pistons 104 therefore cannot be set too close to zero.